The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Knock is the audible sound produced by intense combustion. Repeated knocking events will result in elevated surface temperatures and surface vibration of the combustion chamber and destructive removal of metal from the piston, cylinder head, valves, spark plug, and cylinder walls.
Although, auto-ignition and detonation have very specific definitions. These terms are used to describe a rapid heat release process of the end gases in the combustion chamber, which is the result of a rapid pressure rise that causes the end gases in the chamber to self ignite (auto-ignite) spontaneously throughout their volume resulting in an explosive combustion or detonation. A rapid pressure rise in the end gases is caused by a pressure wave that is traveling faster than the flame front, which results in the compression of the end gases and a rise in the end gas temperature that is sufficient to result in a spontaneous combustion or detonation of the entire volume of end gases. This results in an instantaneous release of heat that causes the cylinder pressure to resonate at the natural acoustical frequencies of the chamber. The sustained oscillations of the pressure waves cause the metal surfaces of the chamber to vibrate and produce the audible sound of knock. Thus, knock is the impulse response of the chamber in response to the rapid pressure rise or heat release that acts as an impulse to trigger the resonances of the combustion chamber. It is the equivalent of hitting the chamber with a hammer to provide an impulse to excite the natural resonant frequencies of the structure of the combustion chamber.
Hence, the rapid heat release and auto-ignition causes an audible effect, which is the knock (i.e. the rapid heat release and auto-ignition are the cause and the knock is the effect). The focus of knock control systems used in production has been to use the pressure oscillations as sensed through an accelerometer that measures the vibrations transmitted to the block structure as a result of the oscillating pressure wave in the combustion chamber. The energy of the oscillations in the block vibrations is used as an index of the intensity of the knock. The knock intensity can be detected by several approaches, such as the integral of the square of the oscillation waveform or the maximum peak-to-peak value of the oscillations. Then, this knock intensity signal is used to retard the spark to the point that the knock disappears. It takes significant cylinder pressure oscillations to be transmitted through the structure of the block and to be detected by the vibration sensor (accelerometer). Thus, the engine must produce significant knock before corrective action is taken to stop it by retarding the spark, which slows down the rate of combustion and prevents the triggering of knock. Hence, low levels of knock intensity are not detected by this method.
Alternatively, cylinder pressure has been used to detect knock by directly detecting the oscillations in the cylinder pressure. Similar to the production method, the energy of the oscillations is used as an index of the knock intensity. This method has the advantage over the block vibration method in that it can detect low levels of knock intensity to provide earlier detection. However, the knock intensity measured for a single combustion event is affected by the location of the sensor in the combustion chamber, and by vibrations in the block from the valve train or other mechanical components. Thus, there is a need for more robust methods of detecting knock as will be described below.